This summer, 61-year-old distance swimmer Diana Nyad will be attempting a 103-mile swim through the shark-populated waters separating Cuba and Florida — and she'll be doing it without a shark cage. Instead, the marathon swimmer will be accompanied by an electronic shark-deterrent device called Shark Shield, which works by screwing with the electrosensory system sharks use to detect bioelectric fields given off by living tissue.

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Housed within the snouts of predatory sharks are special sensing organs known as the ampullae of Lorenzini. The organs are electroreceptors, and they allow sharks to sense electromagnetic fields and temperature gradients in their surrounding waters. A shark's ampullae can prove particularly useful when it is searching for prey. All this is well and good for the shark, but it's bad news for Nyad, whose contracting muscles will be giving off more than enough energy to attract the attention of a few hungry sharks.

To stave off any curious, cartilaginous fishes in her immediate vicinity, Nyad will be accompanied by a number of Shark Shields like the one pictured here. The shield works like a force field, emitting a three dimensional electrical waveform that projects as far as 8 meters away from the two electrodes where the field originates.

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Christine Ambrosino, a graduate student in the University of Hawaii's Department of Zoology whose research focuses on the electrosensory systems of hammerhead sharks, explains how the devices will protect Nyad on her swim:

[The electrodes] set up such a strong electric pulse in the water that as the shark swims toward it, it's like punching them in the face with a cattle prod...If you're out there and you see a big shark coming, that's security that you probably want to have.

The field does no lasting damage to the shark, but induces uncontrollable spasms in its nose that increase in frequency and intensity as it gets closer to the field's point of origin.

You can read more about Shark Shields and Diana Nyad's record-breaking swim plans, over at Discovery News
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